In recent years, projection display apparatus have been popularized widely.
As one of such projection display apparatus as just mentioned, a projection display apparatus of the back projection type which projects image light to a screen of the transmission type from the rear side of the screen is known.
In such a projection display apparatus of the back projection type as just described, for example, light of a white light source is collimated into a light flux by a reflector a like element, and the light flux is separated into light fluxes of three colors of red, green and blue by color separation mirrors.
Then, the light fluxes of the three colors are introduced into two-dimensional image display elements (for example, LCD units; Liquid Crystal Display units) formed in accordance with image electric signals of red, green and blue (R, G, B). The image lights obtained on the two-dimensional image display elements corresponding to red, green and blue are color-synthesized into white light by a color synthesis optical system and projected in an enlarged scale on a screen of the transmission type through a projection lens.
It is to be noted that, for a lens having a similar configuration, a great number of lenses including photographic objective lenses of the wide angle type for a single-lens reflect camera having a long back focus and wide angle projection lenses for a projection television which uses a CRT (Cathode Ray Tube) have been proposed taking limitations by a quick return mirror and so forth into consideration.
Further, in a projection display apparatus, a lens system which forms a projection lens sometimes has a structure for converting a light path.
The structure makes it possible to change the arrangement direction of a housing of a projection apparatus in a projection apparatus or an arrangement direction of each of various optical elements from the color separation to the color synthesis in the inside of a projection apparatus or miniaturize the various optical elements, thereby achieving miniaturization of the projection display apparatus.
Particularly in projection display apparatus in recent years, since a demand for achievement of higher definition of a projected image and for miniaturization of an entire apparatus is increasing, a wider angle and a higher optical performance are demanded also for a projection lens.
However, with such a configuration of a projection display apparatus as described above, from a limitation arising from the necessity to dispose an optical element such as a dichroic prism or a dichroic mirror as a color synthesis optical system, a rather long back focus which corresponds to the distance from the two-dimensional image display element to the last end of the projection lens must be assured.
Where a projection display apparatus forms an enlarged image on an entire screen of the transmission type using a single projection apparatus, in order to make the projection display apparatus itself compact, it is necessary to reduce the projection distance (typically the length of a center light beam from an outgoing end of the projection lens to the transmission type screen through a mirror). To this end, it is necessary to form the projection lens as a wide angle projection lens to increase the angle of divergence of outgoing light from the projection lens to obtain a large image plane.
Further, in order for the two-dimensional image display element to pass light incoming from the light source therethrough to project an image on the two-dimensional image display element in an enlarged scale and in a high contrast on the screen, it is necessary to utilize a light flux which comes out at an angle proximate to a perpendicular from the two-dimensional image display element.
Accordingly, the projection lens must have a telecentric property so that an off-axis principal ray of the projection lens may be perpendicular to the two-dimensional image display element.
Further, while a display device such as an LCD unit is normally used as the two-dimensional image display element, since the LCD unit is driven using matrix electrodes, it is difficult to correct the distortion of the projection lens, different from an alternative case wherein a CRT is used. In particular, where a CRT is used, the distortion of the projection lens can be corrected comparatively readily by utilization of a raster-shape correction function such as bobbin-like distortion correction. However, where a display device which displays in a dot matrix like an LCD unit is used, such raster distortion correction as described above is not performed usually.
From such a situation as described above, preferably the distortion aberration of the projection lens is minimized. However, this makes an obstacle to formation of the projection lens as a wide angle lens and achievement of a long back focus.
In short, it is known that, if a telecentric property is provided to the projection lens while a wide angle and a long back focus are assured, then this provides a tendency that the overall length of the lens increases or the lens diameter increases.
Further, if the lens system which forms the projection lens adopts a structure which converts the light path as described above, then it is possible to suppress the height and reduce the depthwise dimension of the projection display apparatus. As a result, miniaturization of the projection display apparatus can be achieved. In recent years, however, further miniaturization is demanded also for a projection display apparatus which includes a projection lens which performs such light path conversion as described above. Also increase in definition of a projected image is promoted.